We propose to demonstrate that acutely administered haloperidol, a dopamine-blocking drug, will cause subjects to increase cigarette smoking and bromocriptine, a dopamine-mimicking drug, will decrease smoking. Research in animals has suggested that dopamine plays a key role in nicotine reinforcement. Theoretically, dopamine is a key neurotransmitter in the drug mediated reward mechanism and is released by nicotine. Cigarette smoking exemplifies stimulant drug reinforcement, similar, in an attenuated way, to cocaine and amphetamine use. Understanding the role of neurotransmitters in the reward mechanism underlying nicotine seeking may help to design a more rational cessation treatment for smoking, potentially by modification of the reward mechanisms that reinforce cigarette smoking. The experiments will follow up two pilot studies conducted with human subjects which showed increased rates of smoking when subjects were given single low doses (0.5 to 2.0mg) of oral haloperidol at one week intervals. In the second pilot, subjects also showed greater desire to smoke with haloperidol than with placebo and there was a trend toward higher serum nicotine levels with haloperidol than with placebo. Smoking related changes with haloperidol did not appear to be attributable to effects of haloperidol on mood or on movement (e.g., akathisia). Both experiments will use repeated measures designs. Subjects will be their own controls. In the first study, 40 subjects will be given four doses of haloperidol (0.0,1.0, 2.0, and 3.0mg) at one week intervals to examine the effects of dopamine blockade on rate of smoking (cigarettes per hour, number and duration of puffs and interpuff interval), desire to smoke, and serum nicotine levels. Subjects will complete mood measures and movement tests. ANOVA and ANCOVA will be used to test for the expected increases in measures of nicotine seeking. In study 2, 40 subjects will be given four doses of bromocriptine (0.0, 1.25, 2.50, and 3.75mg), a dopamine agonist, at one week intervals. ANOVA and ANCOVA will be used to test for expected decreases in nicotine seeking and desire to smoke. Mood measures will be administered.